Electric machines are utilized in a wide variety of applications. For example, hybrid/electric vehicles (HEVs) typically include an electric traction drive system that includes an alternating current (AC) electric machine that is driven by a power converter with a direct current (DC) power source, such as a storage battery. Machine windings of the AC electric machine can be coupled to inverter sub-modules of a power inverter module (PIM). Each inverter sub-module includes a pair of switches that switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. This AC power drives the AC electric machine, which in turn drives a shaft of the HEV's drivetrain.
As used herein, the term “multi-phase” refers to two or more phases, and can be used to refer to electric machines that have two or more phases. A multi-phase electric machine typically includes a multi-phase pulse width modulated (PWM) inverter module that drives one or more multi-phase AC machine(s). One example of such a multi-phase electric machine is a three-phase permanent magnet AC machine. In a three-phase system, a three-phase PWM inverter module drives one or more three-phase permanent magnet AC machine(s). For example, some traditional HEVs implement two three-phase PWM inverter modules and two three-phase permanent magnet AC machines each being driven by a corresponding one of the three-phase PWM inverter modules that it is coupled to.
In many conventional motor drive systems, the inverter modules are driven by switching vector signals that are generated based on voltage command signals. For example, in a conventional motor drive system that relies on closed loop current control techniques, these voltage command signals can be generated based on feedback or measured stator currents and current commands that are processed by a current regulator.
One drawback associated with machine drive systems that are used to drive permanent magnet machines is that when they undergo an abrupt change from one operating point to another, large transient currents can be introduced. For example, when transitioning from some initial condition to three-phase short abruptly, a large transient current is introduced. The transient current amplitude can easily equal or even exceed two times the motor characteristic current. The transient current presents significant stress to both the motor and inverter. Additionally, the transient current often peaks close to the negative d-axis of the machine. The negative d-axis current tends to oppose the permanent magnet flux. If the current is sufficiently large, it can result in demagnetization of the rotor magnets.
One of the highest performing magnets being used today is the rare earth NeFeB type. Several additives are used to enhance the properties of the magnet. One of these is Dysprosium, which increases coercivity and enhances the robustness of the magnet to demagnetization. Unfortunately, Dysprosium is very expensive. If the large transient negative d-axis current can be eliminated, then it is possible to reduce the Dysprosium content and use lower grade magnets without fear of demagnetization. Thus, the machine cost can be significantly reduced.
Other machine designs employ ferrite type magnets which are lower cost. However, these magnets are also particularly susceptible to demagnetization. It would be desirable to reduce or eliminate the threat of demagnetization by reducing the large transient negative d-axis current since doing so would allow for lower cost designs to be employed.
Finally, regardless of magnet type, in any permanent magnet machine design, the peak negative d-axis current is considered as a design constraint. The designer must optimize the rotor geometry to avoid demagnetization of the magnet under the expected peak negative d-axis current. If the peak current amplitude can be reduced, then this eases the design constraints due to demagnetization concerns, possibly allowing for improved torque density and/or efficiency.
It would be desirable to provide improved methods, systems and apparatus for generating voltage commands used to control a multi-phase permanent magnet machine. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.